Language switcher link: domain.com/switch_language/1/457/1204
"The main problem is that 302s and meta refreshes, are poor substitutes, as they generally will not pass the rankings and search engine value. The only time these redirects are good alternatives is if a webmaster purposefully doesn't want to pass link equity from the old page to the new -https://moz.com/learn/seo/redirection."...
What can I tell him not to worry about this?
Which template are you using for the Switch Language block that's displaying the languages?
Since concrete5 version 8.2.0 the "Flags" template uses URL of pages instead of redirects.
If you are using a template from a 3rd party theme/package you should update its code.